pH-dependent stability of neuroserpin is mediated by histidines 119 and 138; Implications for the control of -sheet A and polymerization

Didier Belorgey 1 *, Peter Hägglöf 1, Maki Onda 2, David A. Lomas 1

1Department of Medicine, University of Cambridge, Cambridge Institute for Medical Research, Cambridge CB2 0XY, United Kingdom 2Department of Biological Science, Osaka Prefecture University, Nakaku, Sakai, Osaka 599-8570, Japan

email: Didier Belorgey (db301@cam.ac.uk)

^*Correspondence to Didier Belorgey, Department of Medicine, University of Cambridge, Cambridge Institute For Medical Research, Cambridge CB2 0XY, UK

Funded by:
 Medical Research Council (UK)
 the Engineering and Physical Sciences Research Council (UK)
 Papworth NHS Trust (UK)
 Swedish Society for Medical Research (Sweden)
 Wenner-Gren Foundation (Sweden)

Neuroserpin is a member of the serpin superfamily. Point mutations in the neuroserpin gene underlie the autosomal dominant dementia, familial encephalopathy with neuroserpin inclusion bodies. This is characterized by the retention of ordered polymers of neuroserpin within the endoplasmic reticulum of neurons. pH has been shown to affect the propensity of several serpins to form polymers. In particular, low pH favors the formation of polymers of both 1-antitrypsin and antithrombin. We report here opposite effects in neuroserpin, with a striking resistance to polymer formation at acidic pH. Mutation of specific histidine residues showed that this effect is not attributable to the shutter domain histidine as would be predicted by analogy with other serpins. Indeed, mutation of the shutter domain His338 decreased neuroserpin stability but had no effect on the pH dependence of polymerization when compared with the wild-type protein. In contrast, mutation of His119 or His138 reduced the polymerization of neuroserpin at both acidic and neutral pH. These residues are at the lower pole of neuroserpin and provide a novel mechanism to control the opening of -sheet A and hence polymerization. This mechanism is likely to have evolved to protect neuroserpin from the acidic environment of the secretory granules.